Click the image above for pages which explain the basic symmetry operations – Proper Rotations, Reflections, Improper Rotations and Centres of Inversion.
Care must be taken not to confuse (a) an inversion operation with (b) a two-fold rotation. Although the two operations may sometimes appear to have the same effect, that is not the case in general. An S1 axis is equivalent to a mirror plane and an S2 axis is equivalent to a centre of inversion.
